The preparation of compositions based on thermoplastic polymers and vulcanized rubbers using a dynamic vulcanization method is known in the art and is described in particular in U.S. Pat. Nos. 3,578,643; 3,862,106; 3,037,954 and 3,806,558.
According to this method, vulcanization of the elastomeric component is made to occur during the step of mixing or mastication with a plastomeric polymer in the molten state, using cross-linking agents, which are usually of the conventional type, such as e.g. peroxides, azide compounds, mixtures of sulphur with zinc oxide, substituted ureas, thiocarbamates, etc. If in the so-treated mixture there is a sufficient amount of plastomeric polymer, the composition, on conclusion of the vulcanization of the elastomeric component, retains good processability characteristics in hot conditions also if it contains high proportions of vulcanized elastomer.
According to French patent No. 2,408,632, polyolefinic plastoelastomeric compositions are obtained using dynamic vulcanization methods. These utilize, as a cross-linking agent, a halogenated phenolic resin, or a phenolic resin which are not halogenated but are associated with halogen donors, in combination with a metal oxide (activator).
According to the patent, the mix of EPDM rubber and olefinic polymer, wherein the phenolic resin content ranges from 5 to 20% for the rubber, is subjected to mastication at a temperature sufficient to melt the olefinic polymer. Then the activator is added and mastication is continued at a temperature at which the elastomeric component cross-linking occurs. The metal oxide, in particular the zinc oxide, favors in this case the complete cross-linking of the elastomer, which is one of the objects of the process of the patent.
Generally, the products obtained according to the above-cited processes exhibit good elastomeric characteristics at room temperature. However, these characteristics, in particular the compression set values, decay as the temperature rises, which jeopardizes the practical utilization of the products above the melting point of the plastomer.
It is further known from U.S. Pat. No. 3,957,979 to prepare thermoplastic elastomers by subjecting a blend of an EPDM interpolymer with polypropylene and substantial amounts of polyethylene to a cross-linking reaction using free radical generating agents, while submitted to hot working.
According to the preparation, polyethylene is preferentially cross-linked and grafted to the EPDM interpolymer, thereby protecting polypropylene from radical attack, degradation and cross-linking or grafting reactions. The product thus obtained possesses improved molding characteristics, but exceedingly loose elastic properties such as elongation as break tension set and compression set, especially at high temperatures.
The above references are hereby incorporated by reference.